The present invention relates to an ionization chamber with a non-radioactive ionization source, preferably of an ion mobility spectrometer, an electron capture detector, or a mass spectrometer with ionization at atmospheric pressure (APIMS), with a reaction compartment, a supply line to feed analyte into the reaction compartment, and a discharge line to remove the analyte, whereby the reaction compartment is separated from an evacuated compartment by a partition which is impervious to gas, whereby a non-radioactive electron source is installed in the evacuated compartment, and forms the negative pole of an acceleration section.
Such an ionization chamber is known from U.S. Pat. No. 5,969,349 for an ion mobility spectrometer (IMS) and from U.S. Pat. No. 6,023,169 for an electron capture detector (ECD).
Ion mobility spectrometers (IMS) were introduced in the early 1970s in order to analyze and detect organic vapors in air. An ion mobility spectrometer consists of the reaction chamber in order to generate ions of substances to be analyzed, and a drift chamber in order to separate the ions. In the reaction chamber radioactive materials are normally used to generate the ions to be analyzed, e.g. tritium, 63Ni, 241Am etc. The disadvantage of such an IMS is that the use of a radioactive ionization source can be hazardous for the environment and the health of the maintenance personnel.
In this connection a large number of attempts were made to design IMS setups with non-radioactive ionization sources in the reaction chamber, e.g. photo-emitters for the generation of electrons. However, in these experiments it was not possible to rule out contact between analyzed gas molecules and the surface of the source. This is one of the reasons for instability in detector displays because such contact can alter the operating characteristics of a non-radioactive source.
Known IMS setups consist of a reaction chamber, a drift chamber, a non-radioactive electron source which is integrated into the said reaction chamber, a supply line connected to the reaction chamber in order to feed an analyte, and a discharge line to remove the analyte, as well as a capture electrode integrated into the drift chamber (for example, see Begley P., Carbin R., Fougler B. F., Sammonds P. G., J. Chromatogr. 588 (1991) Page 239).
The disadvantage of this known IMS is that the analyte makes direct contact with the surface of the non-radioactive ionization source, which in turn alters the operating conditions of the said ionization source and can be one of the reasons for instabilities in detector display.
U.S. Pat. No. 5,021,654 describes how a radioactive ion source can be simply replaced by a non-radioactive one in the form of a thermionic emission source.
With the ionization chamber referred to at the beginning it is possible to create an IMS or ECD setup which avoids contacts between the analyte and the ionization source and permits operation with positive and negative ions.
Due to the fact that the electron source is accommodated in a separate, evacuated compartment, all contact between the gas and its surface is avoided and prevailing operating conditions are always uniform and controlled. On the other hand, the transparency of the partition for electrons makes it possible for them to pass into the second compartment of the reaction chamber, which forms part of the IMS gas circuit and where, after the electrons have entered through the partition, molecule ions are formed for positive or negative IMS operating modes, by means of reactions with the gas molecules. In a preferred embodiment the partition which divides the reaction chamber into two compartments is made from mica. This is a particularly suitable material both with a high level of electron transparency and sufficient imperviousness to gas. To avoid any bending in the partition due to differences in pressure, it should preferably be supported by a metal mesh, e.g. made from copper, with minimal scatter and absorption of electrons.
Although the known reaction compartment already solves a range of problems, there is still the serious problem that, for the partition to be adequately transparent for electrons it must be extremely thin. This involves the risk that despite the supporting measures mentioned the window may mechanically break or leak due to the difference in pressure, particularly in light of the additional load exerted by the intensive electron bombardment, which, among other things, leads to a local thermal load which can only be dissipated inadequately via the supporting mesh and an extremely thin metal film. Most of the electrons hitting the partition are still absorbed in the wall and as operation of the electron source progresses they cause irreversible changes to the partition, as a result of which its imperviousness is reduced. Only electrons in the sub-ppm range can penetrate the wall and ionize the air components in the reaction compartment, which is why only small measuring signals occur. Larger measuring signals could be achieved by increasing either the electron current which penetrates the partition or the voltage with which the electrons are accelerated in front of the partition. However, in both cases the energy input into the partition increases and, because the charges which have penetrated are only discharged inefficiently in the wall material (e.g. mica), it brings about a reduction of the life of the apparatus, which can be dramatic, depending on the composition of the wall material.
For this reason there is still the need for an ionization chamber with a non-radioactive source of the type referred to at the beginning and having a sufficient or even higher ionization rate for the required ion molecule reactions in the reaction compartment and, on the other hand, with a stable, vacuum-tight partition with a long service life in operation.
The problem is solved, on the one hand, by an ionization chamber of the type mentioned at the beginning in which the positive pole of the acceleration section is designed as an x-ray anode in the evacuated compartment, possibly as the surface of the partition, such that a) x-ray light generated in the x-ray anode by impinging electrons reaches the partition in the direction of the reaction compartment, b) the partition is essentially impervious to the electrons of the kinetic energy achieved by the acceleration voltage and largely permeable to the x-ray light generated in the x-ray anode, and c) in the reaction compartment, possibly as the surface of the partition, one or more electrodes are installed in order to generate photoelectrons from the x-ray light passing through the partition.
The problem is also solved by an ionization chamber of the type referred to at the beginning in which the positive pole is designed as an x-ray anode in the evacuated compartment, possibly as the surface of the partition, in such a way that a) x-ray light generated in the x-ray anode reaches the partition in the direction of the reaction compartment, b) the compartment is essentially impervious to electrons of the kinetic energy achieved by the acceleration voltage and is largely permeable to the x-ray light generated in the x-ray anode, c) the x-ray light largely comprises quantum energies below 2 keV, and preferably below 1 keV, when entering the reaction compartment, so that in the reaction compartment air constituents are effectively ionized by the x-ray quanta.
The invention also comprises a method for the ionization of air constituents in a reaction compartment at atmospheric pressure, particularly of an IMS, an ECD, or an APIMS, in which x-radiation is generated by electron bombardment in a vacuum outside the reaction compartment and this x-radiation passes into the reaction compartment through a stable, vacuum tight partition, which is largely transparent for the x-radiation generated, where it releases photoelectrons and/or lower-energy x-ray quanta, which ionize the air constituents, on one or more electrodes.
Finally the invention comprises a method for the ionization of air constituents in a reaction compartment at atmospheric pressure, particularly of an lMS, an ECD, or an APIMS, in which x-radiation is generated by electron bombardment in a vacuum outside the reaction compartment, and this x-radiation passes into the reaction compartment through a stable, vacuum-tight partition which is largely transparent for the x-radiation generated, where the quantum energy of the x-ray quanta is less than 2 keV, and preferably less than 1 keV, so that the x-ray quanta ionize the air constituents with adequate efficiency.
In a preferred embodiment of the ionization chamber according to the invention the partition is made from beryllium and has a thickness of between 10 xcexcm and 200 xcexcm. Beryllium windows are essentially known from x-ray equipment and are used due to their good transparency in conjunction with adequate strength. In the preferred thickness range the partition is durable and vacuum tight and represents an impenetrable barrier for electrons. The x-ray light generated in the anode can, on the other hand, pass through the partition virtually unhindered.
A preferred alternative is a partition made from mica with a thickness of between 7 xcexcm and 40 xcexcm. Although mica is not as permeable as beryllium for the x-radiation concerned and the partition has to be thinner, the disadvantages of beryllium, i.e. the higher price and toxicity, are avoided.
In embodiments of the invention the acceleration voltage is between 2 keV and 20 keV, and preferably between 5 keV and 15 keV.
In this way x-radiation can be generated in the x-ray anode which is either directly suitable for ionizing air constituents in the reaction compartment or, by conversion in a conversion layer there, releasing photoelectrons and/or generating lower-energy x-radiation which performs this function.
Preferably the x-ray anode contains elements with atomic numbers higher than 50, particularly gold. Consequently a higher level of bremsstrahlung is generated.
The x-ray anode is preferably placed inside the evacuated compartment at a distance from the partition so that essentially none of the electrons emanating from the electron source reach the partition. This is achieved, for example, by an arrangement where the electrons are accelerated toward the x-ray electrode approximately parallel to the partition, where they hit at approx. less than 45xc2x0 and generate x-radiation (characteristic radiation and bremsstrahlung). Only the x-radiation hits the partition, which is thus not encumbered by electrons.
Alternatively, however, the x-ray anode can be applied to the partition as a metal layer, so the electrons from the electron source which hit the anode are decelerated in the metal layer and generate x-radiation which enters the partition on the opposite side and penetrates it.
The metal layer should preferably be thick enough for it to cover at least 7 half-value layers of the electrons penetrating from the electron source, so that practically no electrons reach the partition direct and the thermal load is already significantly reduced due to the conductivity of the metal layer.
However, on the other hand, the metal layer should be thin enough for it to cover a maximum of two half-value layers of x-radiation generated. This ensures that adequately intense x-radiation penetrates the partition into the reaction compartment.
Preferably the electron source includes a thermionic cathode. This is the most common way of generating electrons. However, the invention can also be used in conjunction with other electron sources.
In one embodiment of the invention the electrode is accommodated in the reaction compartment as a conversion layer on the partition. This is an easily implemented variant. The x-radiation entering through the partition generates in its volume and on its surface, photoelectrons and/or lower energy x-ray quanta which enter the reaction compartment and ionize air constituents there.
The conversion layer should preferably be sufficiently thick for it to cover at least 1 but a maximum of 7 half-value layers of the x-radiation impinging on it. This ensures an adequate level of efficiency for conversion.
In particular the thickness of the conversion layer is between 1 xcexcm and 200 xcexcm, depending on the conversion material used, which can also have more than one constituent, and the energy of the x-ray quanta entering. Conversion to lower-energy radiation and ultimately photoelectrons can take place via several conversion stages, whereby the use of adapted materials is recommended accordingly.
Alternatively, the electrode or electrodes in the reaction compartment can also be positioned at a distance from the partition so that x-radiation hits the conversion layer(s) at an angle.
A further development of this embodiment uses several essentially parallel electrodes in the reaction compartment which are positioned at a distance from the partition, such that x-radiation hits the conversion layers at an angle of about 90xc2x0. As a result the incident x-radiation in the reaction compartment is very effectively converted into radiation and/or electrons which ionize the air constituents with a good level of efficiency.
The individual electrodes should be sufficiently thick for each of them to cover about one tenth to one half-value thickness of the x-radiation hitting them, so that all the electrodes contribute to conversion.
This effect can be further intensified by enlarging the effective areas of conversion if the one or more electrodes is/are placed in the reaction compartment at a distance from the partition and if they have a louvered surface structure.
In one embodiment the conversion layers are comprised of materials, the K-shell levels of which are smaller than the mean quantum energies of the x-radiation hitting them. Therefore secondary x-radiation, which has a lower quantum energy and is therefore better suited to effective ionization, can be generated, in a cascade if required.
In one embodiment the conversion layers are comprised of materials, the K-shell levels of which are approximately the same as the mean quantum energies of the x-radiation hitting them. Therefore photoelectrons are effectively released which ionize the air constituents.
When using several elements in the conversion layers, the two effects just mentioned can also be combined. Lower-energy x-ray quanta are generated, in several stages if required. They either ionize air constituents with an already good degree of efficiency or they then release photoelectrons which cause the ionization.
Between the electron source and the x-ray anode an additional focusing electrode can be placed which is connected to the acceleration voltage source.
By contrast with a xcex2 source, the intensity and/or energy of the electrons, i.e. their range, can advantageously be changed and thereby optimized for the respective conditions, particularly the geometric conditions. In the case of an ECD the electron range can be reduced from about 7 mm for an Ni-63 source to less than 0.2 mm by generating electrons with energy levels of about 1.5 to 2 keV instead of 16 keV for a Ni-63 source, thus considerably reducing the detector volume, for capillary column detectors, for instance, and yet retaining the required, spatially inhomogeneous ionization.
With an IMS the electron range can be adapted to the length of the reaction compartment. This is particularly important in the case of miniaturization (micro-IMS).
By altering the intensity, the sensitivity can be increased or adapted to the respective measurement. If in an upline overview scan or a preceding measurement no product ions are found or only an insufficient quantity, the intensity can be increased correspondingly. Correspondingly, the intensity can be reduced again if the number of product ions is higher than necessary.
Further advantages of the invention are contained in the description and the enclosed drawings. In addition, the above-mentioned, detailed features of the invention can be applied individually or used together in various combinations.
The described embodiments are not to be understood as a conclusive list but, on the contrary, they are examples.
The effects which ultimately lead to the ionization chamber setup according to the invention occurred very surprisingly during experimentation. In the following. there will be a few semi quantitative, more general calculations and estimates in advance, which can provide an initial insight into understanding the background of potential physical mechanisms which are exploited by the invention.
If high-energy electrons penetrate a solid, they are decelerated, whereby their kinetic energy is distributed among new charge carriers (xe2x86x92xe2x80x9cIonization moderationxe2x80x9d) and the generation of radiation (xe2x86x92xe2x80x9cRadiation moderationxe2x80x9d).
When the primary electron collides with an extranuclear electron of the moderating medium, up to 50% of its kinetic energy is transferred. This energy is distributed over the work function (=bonding energy of the extranuclear electron, e.g. approx. 15 eV for a valence electron or approx. 0.5 . . . 1.5 keV for a K-shell electron) and kinetic energy of the resulting secondary electron. If this energy is sufficiently large, ionization processes can take place again.
Apart from these ionization processes, elastic scattering of the primary and secondary electrons also take place. As kinetic energy declines, the angle of deflection (relative to the original direction of movement) becomes larger and larger. Consequently, and due to the basic non-discriminatability of primary and secondary electrons (the higher-energy one is termed primary electron) the electron paths branch out considerably toward their end, i.e. it is not possible to talk about a defined range of the primary electrons.
If one plots the flux density of monoenergetic electrons relative to the thickness of the moderating medium, there is an almost linear decrease as layer thickness increases, the extrapolated intersection of which with the layer thickness axis is referred to as xe2x80x9cmean rangexe2x80x9d. The range is not only stated in x (cm) but also in x xcfx81 (g/cm2) (xe2x80x9cMass rangexe2x80x9d) because as long as the ratio between the atomic number and atomic weight is constant for the moderating medium, the xe2x80x9cmass moderating capacityxe2x80x9d (xe2x88x92dE/dx)/xcfx81 is virtually independent of the type of medium, i.e. the xe2x80x9clinearxe2x80x9d ranges x can be converted between the media (e.g. aluminium-copper-air) taking the respective media densities into account.
In some ionization chambers, windows with a thickness of approx. 6 xcexcm and made from muscovite mica (muscovite=potassium mica=KAI2 ((OH1F)2/AI Si3O10) mean atomic number: 9.4, mean atomic weight: 19, density: 2.6 . . . 3.2 g/cm2, in the following the calculations use a figure of 2.8, i.e. 6 xcexcm=1.7 mg/cm2) were installed which have an external aluminium thickness (i.e. on the air side) of 30 . . . 50 nm.
The electron range in the window material can be estimated according to the following equation:
R=0.5 E(1xe2x88x920.983/(1+4.29 E))=approx. 7 HVT
where R is the electron range in g/cm2, E is the electron energy in MeV, and HVT is the half-value thickness, i.e. the layer thickness which halves the energy of the electrons.
The equation was checked by using 63Ni-xcex2 -radiation (mean energy 16 keV) and air as the moderating medium: 1 HVT=0.9 mm air. Other sources in literature state HVT as being 0.5 . . . 1.3 mm air (mean 0.9 mm).
With this equation the electron ranges (in mg/cm2 and in xcexcm mica) and the HVT (in xcexcm mica) were calculated relative to the electron energy:
Column 5: 6 xcexcm mica corresponds to n HVT
Column 6: Reduction in electron flow after passing through 6 xcexcm mica by a factor of 2(number of HVT).
Below approx. 15 keV and particularly below 10 keV it is highly likely that no primary electrons will pass through the window.
If fast electrons (1 . . . 100 keV) are deflected and decelerated in the Coulomb field of heavy nuclei, the so-called bremsstrahlung occurs, the energy distribution of which ranges from 0 to the maximum energy of the electrons. The intensity peak of the bremsstrahlung spectrum is 1.5 . . . 2 times the short-wave limit, i.e. approx. 10 keV (=1.25 Angstroem) for example if the electrons penetrate the moderating medium at 15 keV (xcexmin=0.83 Angstroem). If the electron energy is larger than the energy of the K-, L-, N- . . . shells of the moderating medium, the continuous bremsstrahlung spectrum is superimposed with the discrete lines of the moderating medium, e.g. in the case of muscovite mica: 3.3 keV from the K, 1.5 keV from the AI, and 1.7 keV from the Si; the radiation with 678 eV from F and 517 eV from 0 (52% of the atoms in mica are 0-atoms!) will probably not be able to leave the window because it has to little energy (radiation absorption xcx9c1/energy).
For the yield of bremsstrahlung various authors quote empirical formulas/characteristics from which the following figures are derived for 15 keV electrons for example:
The bremsstrahlung yields are minimal. Most of the energy of the primary ions is converted to charge carriers (i.e. to secondary electrons) by means of xe2x80x9cionization moderationxe2x80x9d and is lost if these secondary electrons do not have adequate energy to leave the window. From Tab. 2 it can be seen that the bremsstrahlung yield could be increased by about 18 times if the primary electrons were not decelerated in the mica (mean atomic number 9.4) but in gold (atomic number 79).
The bremsstrahlung resulting in the window is attenuated on its journey through the window. This reduction in intensity is described by Lambert-Beer"" law: I/Io=exe2x88x92(xcexc/xcfx81)xxcfx81 with (xcexc/xcfx81)=mass attenuation coefficient and xxcfx81=area density of the attenuating layer (up to 1.7 mg/cm2 for the mica window). The values for mass attenuation coefficient are summarized in various literature in the form of characteristics and tables. Reductions in the intensity of bremsstrahlung depending on the energy of the primary ions (=maximum energy of the quanta) have been calculated (Tab. 3). 13 and 6.5 keV are the peaks on the bremsstrahlung spectra, which are caused by 20 and 10 keV electrons respectively.
From these calculations it is evident that, as already supposed, radiation with less than 1 keV will not be able to leave the window and the characteristic radiation of K, Al, and Si leaves the window but only highly attenuated. Consequently, the spectrum will be limited to the bremsstrahlung xe2x80x9cmountainxe2x80x9d, i.e. to the energy range from approx. 3 keV . . . Emax.
Attenuation of bremsstrahlung in the exterior aluminium layer, which is 30 . . . 50 nm thick, is minimal, as indicated by the figures in Tab. 4. An average thickness of 40 nm=4xc3x9710xe2x88x926 cm is assumed, which, multiplied by the density of the aluminium (=2.7 g/cm2), is equivalent to a mass layer thickness of 1.1xc3x9710xe2x88x925 g/cm2.
The loss in the intensity of the bremsstrahlung in the window is due to interaction between the quanta and the shell electrons of the atoms of the window materials. At low atomic numbers and low quantum energies that is the photo effect.
The photo effect is a pure absorption process. The entire quantum energy Ex is transferred to an electron which then leaves the atom with a kinetic energy of Ekin=Exxe2x88x92Ei, where Ei refers to the bonding energy of the electron in its shell (K. L, M, . . . ). If the quantum energy is larger than E (K) (=bonding energy in the K-shell), photo absorption chiefly takes place (approx. 80%) in the K-shell and only about 20% takes place in higher shells. The probability of the photo effect is highest if the quantum energy is just a little higher than the bonding energy of the electron. The emission angle of the photoelectron (relative to the direction of incidence of the quantum) is dependent on the quantum energy: 11xc2x0 at 11.3 MeV, 43xc2x0 at 79 keV, 65xc2x0 at 17 keV, and xe2x86x9290xc2x0 at even smaller energies.
If the kinetic energy passed on to the photoelectron is larger than the bonding energy of electrons in adjacent atoms, secondary electrons will be released there.
The hole which the photoelectron leaves behind (e.g. in the K-shell) is filled by a more distant electron which, with its jump, releases the path energy differential through radiation. This characteristic x-radiation can again liberate a photoelectron in an adjacent atom (naturally with less energy). However, the so-called intrinsic photoeffect is also possible, in whichxe2x80x94without any radiationxe2x80x94a further (more distant) extranuclear electron of the same atom is emitted (Auger effect): an L-electron fills the hole in the K-shell and passes the energy differential on to the other L-electron, for example, which can consequently leave the atom, whereby the energy imparted is approximately E(K)xe2x88x922E(L) (for aluminium for example : approx. 1,500 eVxe2x88x922xc3x97165 eV=1.2 keV). Such radiation-free transitions are very probable with light elements.
The process of xe2x80x9cfilling holesxe2x80x9d can be continued in a cascade so that a large number of electrons are released which have a wide range of low energies.
The described interaction processes can take place both in the window material and in the air in the IMS ion source.
Feasible methods are described which can lead to saturation currents in the IMS ion source.
The electrons produced and accelerated in the ion source penetrate the window and create bremsstrahlung with a power of 1.2xc3x971014 eV/s (as described in the previous chapters). According to the empirical equation P=1.5xc3x9710xe2x88x929xc3x97Zxc3x97ixc3x97U2, whereby P is the bremsstrahlung power at complete absorption of the electron beam, Z is the atomic number of the moderating medium, i is the electron flow, and U is the voltage to accelerate the electrons, this power can be estimated at 8.8xc3x971013 eV/s (the condition xe2x80x9ccomplete absorption of the electron beamxe2x80x9d can, as was shown, be regarded as fulfilled), the deviation is about xe2x88x9225%. The radiation spreads in a 4xcfx80 geometry; we are only concerned with the hemisphere directed toward the IMS ion source (xe2x86x92factor 0.5). The bremsstrahlung spectrum probably has an intensity peak at approx. 6.5 keVxe2x80x94so this value is used in the further calculation (xe2x86x92factor 0.65). About 20% of the radiation is attenuated in the window (xe2x86x92factor 0.8). In the thin aluminium layer on the outer window surface 0.1% of the radiation is absorbed and converted into photoelectrons. The aluminium electrons are (on average) bound at 170 eV. Consequently, approx. 1.8xc3x97108 photoelectrons result per second, of which only the 50% which leave the aluminium layer toward the IMS ion source (xe2x86x92factor 0.5) are important. The mean energy of these photoelectrons can only be estimated with difficulty. If it is about 1 keV (or slightly more) (e.g. L-shell Auger electrons, see above), these electrons have a chance of leaving the aluminium layer: an average aluminium layer thickness of 40 nm corresponds to 6.8 HVT for 1 keV electrons, i.e. the probability that these electrons leave the window is 1%. A 1 keV electron produces about 3 ion pairs per cm and Torr in air; at 760 Torr and a maximum range of 120 xcexcm (=10 HVT) this produces 27.4 ion pairs per photoelectron or 2.5xc3x97109 ion pairs per second. If one multiplies this figure by the elementary charge, the saturation current will be about 410 pA. On the other hand, the energy input of 9xc3x97107 electrons/sxc3x971 keV/electron into the air of the IMS ion source can also be used to calculate the saturation current by dividing it by the air-specific ionization effort of approx. 34 eV per ion pair and then multiplying it by the elementary charge: 420 pA.
The measured saturation currents are between 170 and 330 pA (depending on the electron source).
As an alternative to the way described here, it is also plausible that the air ionization occurs not via the intermediate step xe2x80x9cphotoelectrons from the aluminum layerxe2x80x9d, but rather directly via the interaction of the bremsstrahlung quanta with the nitrogen and oxygen atoms in the air. In the first step, the form of interaction is the photo effect on the atoms with the formation of photoelectrons, while in the second step it is the ionization of the N2 and O2 molecules by these photoelectrons. Since the quanta are not charged, they have only a low probability of interaction, i.e. the bremsstrahlung quanta (compared with electrons of the same energy) have ranges about 1,500 times greater in air.
In Table 5 the radiation ranges R (1%) are listed with the distances required to reduce radiation to  less than 1% (line 3), as well as the values for reduction of radiation in the reaction compartment (approx. 3 cm long, line 4), and reduction of radiation in the whole IMS measuring cell (approx. 8 cm long, line 5).
If the quanta have energies greater than 3 keV, as estimated above, they should travel through the whole measuring cell with only minimal interaction and impinge on the collecting electrode. This would result in a constant ionization current (caused by the ionization in the drift compartment and by release of photoelectrons in the collecting electrode), which causes an increase in the baseline of the spectrum. However, since this was not observed, one has to conclude that either the quanta do not reach the drift compartment, or that their probability of interaction in the IMS measuring cell is so low that they cause almost no detectable effects. In order to estimate the proportion of radiation which enters the IMS measuring cell (more specifically, the reaction compartment), the ratio of the reaction compartment volume (approx. 2.5 cm3) to the volume of a sphere with a radius R (1%) is introduced as a correction factor.
Table 6 shows the results (ion pairs as well as saturation current) in consideration of the geometric proportions.
From the values in the last line of table 6 one can see that two effects can come into play. If the bremsstrahlung spectrum spans the energy range of approx. 3 keV up to the energy of the primary electrons, air ionization by the bremsstrahlung is hardly probable; if, however, the lower energy component in the spectrum is not to be neglected, then the ionization current caused by the bremsstrahlung can quickly begin to dominate.
The invention is depicted in the diagrams and explained and described using actual embodiments in more detail.